codominance

What if the heterozygous trait exhibited codominance?  in this case, two alleles can code for co-dominant traits.  That is, neither is dominant over the other.  Blood type is an example of this:

A is codominant to B
A is dominant over O
B is dominant over O

Possible genotypes:

heterozygous AB blood gives both type A protein and type  B protein on the erythrocytes.

homozygous AA gives blood type A  protein on the erythrocytes
homozygous BB is blood type B protein on the erythrocytes

Heterozygous AO is gives blood type A
Heterozygous BO is blood type B

 

Sex linkage

 SEX LINKED GENES

In the following post, I will refer to "male" and "female".  This refers to people with male sex organs or people with female sex organs.  People with these organs can identify with a diverse range of gender such as:

Man, woman, nonbinary. This gender identify is not presumed by the presence of male or female sex organs

They might also identify as

LGBTQ2S++

The following descriptions are biological descriptions refer to 

male= male sex organs like testes, producing sperm

female= female sex organs like ovaries, producing eggs. 

The sex chromosomes are XX and XY in humans. Some alleles are located on the X chromosome. But very few on the Y chromosome.  Remember that people who are  females have the genotype XX and people who are males have XY.

The Y chromosome carries very little information, only enough to influence the embryo on the path towards male primary and secondary characteristics.

Meanwhile, many RECESSIVE ALLELES are located on the X chromosome, including
1. hemophilia, the inability to clot blood
2. colourblindness inability to tell the difference between red and green colour
3.  baldness

These characteristics are far more common in people with male sex organs  than people with female sex organs because  people with male organs have only one X. And if that X contains the recessive allele, they will show the phenotype.  Meanwhile,people with  female organs may have the recessive and their dominant genes on their other X will protect them.  For example

An example of using a punnet square to calculate probability is here:

Some things to note about sex linkage:  Signs of sex linked alleles are:
1. more genotypic males get the phenotype, but genotypic females can be carriers
2.  males can inherit from mother but not father (because males get their X from mom)
3.  genotypic females have an extra X and this protects them.  Females must be homozygous to show the sex linked phenotype.
Further reading on sex linkage here http://learn.genetics.utah.edu/content/pigeons/sexlinkage/

FYI if you were born a male and you want to know if you're colourblind:  Test yourself here: But careful, if you are a genetically male, you might find out you are colour blind!  Apparently most people find out they are colourblind from their science teacher !  
http://colorvisiontesting.com/home.html

Basic practice worksheet going over main concepts

Worksheet B